Relative Weights of σ and π Ring Currents in a Few Simple Monocycles.

By partitioning the bond current strength (current susceptibility) into plane symmetric and plane antisymmetric contributions, it is shown that 91% of the diatropic ring current of benzene is transported by the π electrons and the remaining non-negligible 9% is sustained by the σ electrons. In planar cyclooctatetraene 94% (6%) of the paratropic ring current is transported by the π (σ) electrons. In cyclopropane 95% (5%) of the diatropic ring current is transported by the σ (π-like) electrons. The 85% fraction of the diatropic ring current of Al4(2-) is transported by the σ valence electrons and 15% by the π valence electrons. In the nonaromatic borazine system the nitrogen-centered π electron circulations are surrounded by a weak diatropic "ring current" 6.5 times smaller than that of benzene.